1. Field of the Invention
This invention relates to methods of and apparatus for sintering tubular ceramic articles and is concerned particularly with the construction and use of a pass-through furnace for this purpose.
2. Prior Art
Pass-through sintering furnaces find particular application in firing ceramic articles where the duration of firing and the firing conditions must be carefully controlled. Such furnaces are used for example in the manufacture of beta-alumina ceramic tubes for use as a solid electrolyte in electrochemical cells and energy conversion devices. As is described for example in U.S. Pat. Nos. 3,922,176; 3,950,463; 4,059,663 and 4,070,542, beta-alumina ceramic tubes may be formed as a green body and sintered by passing them rapidly through a furnace having a sintering zone which may be short compared with the length of each article. The article may be passed through such a furnace quite quickly so that the sintering time is typically less than 2 minutes. It is important that the article, if it is to be uniformly treated over its whole length, should be moved at a substantially uniform speed through the sintering zone of a furnace. For tubular articles, the furnace preferably has a rotatable firing tube through which the articles pass, these articles being moved longitudinally through this tube but allowed to roll as the furnace tube rotates. This rotation of the furnace tube gives improved uniformity of sintering conditions and improved straightness of the articles being fired. With such a technique, the articles cannot be carried through the furnace on a conveyor system and heretofore the technique that has been employed is to push the articles into and through the furnace. To give uniform firing, a succession of articles, possibly separated with spacers, can be pushed through the furnace. Typically beta-alumina ceramic tubes, such as are used in sodium sulphur cells as a solid electrolyte, might be fed into and through the furnace at 80 mm per minute.
The article before it enters the furnace is a green shape, essentially a pressed compact of powder material. As the shape passes through the furnace and is heated up, it becomes softer and slightly plastic and one of the problems which occurs in the firing of ceramic tubes, using a push-through technique as described above, is that the shape of the tube may be distorted by the loads induced by pushing of a series of tubes. In particular, if an open-ended tube is pushed, rubbing occurs at the open end as the tube is pushed and rotated. Furthermore tubes may tend to bend slightly as they soften when heated. Tubes of this nature are commonly made with one closed end, this end being domed to give adequate strength. Pushing on the domed end gives a localised load which again may distort the shape of the tube.